The invention concerns a method of NMR (=nuclear magnetic resonance) tomography (=MRT) for generating NMR gradient echo signals according to the principle of signal generation in the driven equilibrium (DE) or also called steady state free precession (SFP) wherein a periodic sequence of radio frequency pulses with a flip angle xcex1 is applied with a time delay TR, wherein the phase of these radio frequency pulses is alternated in subsequent steps.
A SFP signal is generated by a continuous sequence of radio frequency pulses and was introduced by Carr as early as 1958 (Phys. Rev. 112, 1693 (1958)). Carr was able to show that implementation of the method with equidistant radio frequency pulses with constant amplitude and alternating phase produces a particularly high signal intensity of the SFP signal of on-resonance spins.
In 1986, this principle was converted by a FISP method (in the meantime called true FISP) into a method of MR imaging (A. Oppelt et al. electromedica 54, 15 (1986)). All gradients are switched such that their integral from the center of a pulse to the center of the subsequent pulse is zero. Subsequent pulses have flip angles xcex1 and alternating phases: P1, P3, P5 . . . =xcex1, P2, P4, P6 . . . =xe2x88x92xcex1. The temporal separation between 2 pulses is called repetition time TR.
The problem with implementation is thereby the fact that the transition into the resulting signal steady state is effected only gradually within a time period determined by T1 relaxation. Until this steady state has been reached, periodic signal fluctuations occur which produce strong image artefacts when using the sequence for MRT (see FIG. 2).
Suppression of this initial signal fluctuation is achieved in that before the continuous sequence of radio frequency pulses, one single pulse with a flip angle xcex1/2 is applied with a time delay of TR/2 (Deimling, M. DE 44 27 497 A1). This suppresses the initial signal modulations and merely a monotonic signal change into the steady state takes place (FIG. 3).
Suppression of signal modulation is explained on the basis of observation of the subsequent signals according to FIG. 4, wherein the radio frequency pulses are chosen to be applied each with a radio frequency field with a phase parallel to the y-axis of the transverse plane. The diagram of transverse magnetization Mx vs. Mz shows that the magnetization vector of the steady state magnetization Mss is tilted relative to the z-axis by an angle xcex1/2 such that Mss of subsequent radio frequency pulses is flipped between +xe2x88x92Mss. Initialization with xcex1/2 brings the original z-magnetization M0 to the correct tilting angle and the magnetization vector is transferred to Mss in subsequent radio frequency pulses corresponding to T1 and T2 relaxation wherein the signals (=absolute amount of the Mx-magnetization) decay monotonously towards Mss(x) and show no modulation.
This is true only for so-called on-resonance spins which experience no phase-change during TR. In MR tomography applications (=MRT) this condition is not met even for very small repetition times TR wherein TR is determined substantially by the switching speeds of the magnetic field gradients.
The magnetic field homogeneities dephase the spins by a phase angle of xcex94xcfx86 between two excitations. With TR=4 ms, xcex94xcfx86=90xc2x0 for an off-resonance frequency is e.g. xcex94xcexa9 of xcex94xcfx86/(TR* 360xc2x0)=66 Hz. This corresponds to an inhomogeneity of 1 ppm at a resonance frequency of 63 MHz at 1.5 tesla field strength. Such inhomogeneities cannot be avoided in applications on humans due to the occurring susceptibility effects.
FIG. 5 shows the signal development in a method optimised according to DE 44 27 497 A1 as a function of xcex94xcfx86. It can be clearly seen that spins with xcex94xcexa9 unequal 0 experience modulation over the first excitation periods.
The corresponding signal intensities for xcex94xcfx86=0xc2x0, 180xc2x0 and 360xc2x0 are shown in FIG. 6. Transfer of the modulations into the steady state amplitude which is characteristic for xcex94xcfx86 is very slow. These modulations produce image artefacts. The behavior differences of the spins which are characterized by xcex94xcfx86=0xc2x0 and xcex94xcfx86=360xc2x0 can be explained in that these spins are mutually dephased by 180xc2x0 in the initial period TR/2 according to the method of DE 44 27 497 A1.
A further disadvantageous property of the method according to DE 44 27 497 A1 consists in that application of the small flip angle xcex1/2 for initialisation of the steady state sequence renders access to only a relatively small part of the originally present magnetization M0 corresponding to M0 sin xcex1/2).
In contrast thereto, it is the object of the present invention to further improve a method of the above-described type such that the above-discussed disadvantages can be eliminated.
In accordance with the invention, this object is achieved in an effective fashion, in that the periodic sequence of radio frequency pulses is preceded by a sequence of (n+1) radio frequency pulses with the following valid conditions:
a first excitation pulse with preferred flip angle xcex10=90xc2x0 precedes the subsequently equidistant sequence of radio frequency pulses at a preferred separation TR/2,
the flip angle xcex11 of the subsequent radio frequency pulse is larger than xcex1 and equal or approximately equal to 180xc2x0,
the flip angle xcex1i of the i-th radio frequency pulse in the region i=2 . . . n is selected such that xcex1i is smaller or equal to xcex1i-1 and larger or equal to xcex1 and
the phases of these radio frequency pulses alternate.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and explained in more detail with reference to embodiments.